In a Long-Term Evolution (LTE) cellular network, as defined in standards set by the 3rd Generation Partnership Project (3GPP), coverage for User Equipment (UE) devices is provided by a Radio Access Network known as the Evolved UMTS Terrestrial Radio Access Network (E-UTRAN). Access points or base stations within the Radio Access Network, that provide service to UE devices over the air interface, are known as Evolved NodeBs (eNodeBs or eNBs). A UE device within a cell may be in a Radio Resource Control (RRC) Idle state or in an RRC Connected state. When the UE device is in the RRC Connected state, it is either uplink (UL) synchronized or not UL synchronized with the cell.
The UE must be synchronized in the uplink in order to be able to transmit signals to the eNodeB over the air interface, except for transmissions on the Physical Random Access Channel (PRACH). That is, the uplink of the UE is time synchronized with the frame structure in the cell.
The eNodeB has some control over the UL synchronization state of the UEs in a cell served by that eNodeB. Specifically, the UL synchronization state of the UEs in the cell can be controlled to some extent by using the parameter timeAlignmentTimer, which is defined for example in the standards document 3GPP TS 36.321 V12.2.1, section 5.2.
Specifically, a UE is UL synchronized if the time alignment timer is running, but the UE loses UL synchronization when the time alignment timer expires. Thus, the UE is only allowed to transmit any signals other than a preamble when the time alignment timer is running. The time alignment timer starts or restarts when the UE receives a time alignment command. A time alignment command is ordering the UE to use the timing advance that results in UE transmissions that are time aligned with transmissions from other UEs in the cell. The time alignment timer is thus used to control how long the UE is considered to be uplink time aligned after the reception of a time alignment command. However, the time alignment timer will never expire if it is set to zero, and hence the UE will never lose UL synchronization.
The UE can receive the time alignment timer in different ways. The parameter timeAlignmentTimerCommon is transmitted by the eNodeB as part of System Information Block Type 2. The UE is able to read this information, and then sets its time alignment timer based on the value of parameter timeAlignmentTimerCommon when it performs an RRC Connection Establishment or an RRC Connection Re-establishment. However, the UE does not use the parameter timeAlignmentTimerCommon to set its time alignment timer in other cases, i.e. the time alignment timer of a UE is not changed when the value for the parameter timeAlignmentTimerCommon is changed while that UE is already in the RRC Connected state.
The parameter timeAlignmentTimerDedicated is part of the Medium Access Control (MAC)-MainConfig information element. This can be included in the set up procedure as part of an RRC Connection Establishment or during an RRC Connection Reconfiguration. Thus, the UE uses the parameter timeAlignmentTimerDedicated to set its time alignment timer.
When a UE is synchronized in the uplink, it requires various semi-static resources on the Physical Uplink Control Channel (PUCCH), for example for Channel Quality Indication (CQI) related reports, Scheduling Request (SR) signals used by the UE for requesting Uplink-Shared Channel (UL-SCH) resources for new transmissions, and Sounding Reference Signals (SRS) used by the eNodeB to estimate the uplink channel conditions for each user to decide the best uplink scheduling.
Therefore, when a UE loses its uplink synchronization, it releases any semi-static PUCCH resources that it may have. These PUCCH resources are only available in a limited amount.
Therefore, allowing RRC Connected UEs to become unsynchronized in the uplink, and to release the semi-static PUCCH resources, increases the number of UEs that can be kept in the RRC Connected state, compared with the situation where all RRC Connected UEs are kept synchronized in the uplink, with allocated semi-static PUCCH resources.
In one existing example, cells are configured with the time alignment timer value set to a default value of zero, so that all RRC Connected UEs are kept synchronized in the uplink. As mentioned above, this sets a constraint on the number of UEs that can be allowed to remain in the RRC Connected state. The existing example allows cells to be manually configured to have a non-zero value for the time alignment timer, allowing RRC Connected UEs to become unsynchronized in the uplink.
3GPP Technical Specification TS 36.331 V11.2.0, chapter 6.3.2 “Radio resource control information elements” contains a description of the timeAlignmentTimer and how the UE uses the timer value.
Allowing RRC connected UEs to become UL unsynchronized has the disadvantage that there is a higher probability of call failure when the unsynchronized UE attempts to start uplink data transmission than when a synchronized UE attempts to start uplink data transmission. This may be caused by failure of the resynchronization procedure or failure of the following RRC Reconfiguration procedure that allocates the semi-static PUCCH resources to the UE.